THE GROWTH AND NONLINEAR EVOLUTION OF HELICAL PERTURBATIONS IN A SWIRLING JET MODEL

The growth and nonlinear evolution of a helical perturbation is invest igated in a simplified swirling jet model, consisting of a line vortex along the axis surrounded by a jet shear layer with both azimuthal an d streamwise vorticity. Inviscid Lagrangian vortex dynamics simulation s demonstrate the mechanisms of vorticity concentration, reorientation , and stretching, as well as the nonlinear interaction and competition between a centrifugal Rayleigh instability and a Kelvin-Helmholtz ins tability feeding on both components of the base flow vorticity. The no nlinear evolution resulting from the interaction of these two instabil ities allows for very different flow behaviors to emerge, depending on whether the helical perturbation wave and the vortex lines of the jet shear layer wind around the jet axis in the same or in opposite direc tions. Large-scale vortex helices evolve that can contain azimuthal vo rticity either of the same or of opposite sign to that initially prese nt in the jet shear layer. These different evolutions are triggered by the differences in the direction of the strain field set up by the ev olving large-scale helix. In both cases, the generation of both signs of azimuthal vorticity due to the centrifugal Rayleigh instability all ows for the possibility of unlimited growth of the helix circulation, in the absence of viscous diffusion. (C) Elsevier, Paris.